Novolak phenolic resins are thermoplastic resins. Various means are known for increasing the weight average molecular weight of novolak phenolic resins. For example, JP-A 2005-306987 describes a two-stage reaction method designed to increase the weight average molecular weight while reducing phenol dimer. JP-A 2004-292791 discloses a method of preparing triazine-modified novolak phenolic resin designed to increase the weight average molecular weight while reducing unreacted phenol. These methods are not regarded versatile since they are limitative in that polymerization in the presence of phosphoric acid is essential in the first stage of synthesizing novolak phenolic resin. In addition, the second stage of reaction needs pH control and addition of aldehyde for the purpose of reducing the phenol dimer or unreacted phenol. The methods are more limitative in this sense too. They are not regarded as simple means for increasing the molecular weight of novolak phenolic resins. Also JP-A 2000-309617 discloses a condensates of phenol derivative, aldehyde and triazine derivative as a phenol-amino condensation resin. This method is also limitative in that polymerization is carried out while restricting a molar ratio of a methylol group content to a methylene bond content in a certain range and effecting reaction in the co-presence of a triazine derivative and aldehyde. This is not regarded simple or brief in increasing the molecular weight of novolak phenolic resins.
On the other hand, resol phenolic resins are thermosetting resins. JP-A H03-243613 discloses a method of preparing a phenol-melamine co-condensation resin. However, this method is applicable to the novolak phenolic resins with difficulty because the synthesis of a phenolic resin as the starting resin is different between the novolak type and the resol type.